A filter and processing sequence is described that efficiently combines and performs two or more tasks required to demodulate a composite 3G (third generation) wireless signal formed by a combination of wideband 3.84 MHz (Universal Mobile Telecommunications System, hereinafter referred to as “UMTS” or Universal Mobile Telecommunications System Terrestrial Radio Access, hereinafter referred to as “UTRA”) carriers and narrowband 1.2288 MHz CDMA-2000 carriers. The three tasks, applied to each spectral component of the 3G wireless signal and described in the order of a traditional filtering structure are: Spectral translation, Bandwidth Reduction, and Sample Rate Selection. These tasks are traditionally implemented in three distinct pieces of hardware or software modules.
The spectrum processed by the receiver is shown in FIG. 1 while a block diagram of a traditional digital receiver that processes a signal with this spectrum is shown in FIG. 2. Note that the center frequencies of the wide-band signals are at multiples of 5.0 MHz while the center frequencies of the narrowband signals are at multiples of 1.25 MHz. The sample frequencies, shown in parenthesis and indicated on the block diagram, are typical and can be changed within wide limits and are presented here for purpose of discussing a specific example. Particular implementation efficiency is to be had when the sample rate is selected to be a rational multiple of the spectral spacing between band centers. This condition makes the spectral down conversion particularly simple. Similarly, implementation efficiency is also to be had when the sample rate is selected to be a rational multiple of the desired output sample rate. This condition makes the alignment of time samples with signal epochs particularly simple. Neither condition is a requirement for the process described here, since an arbitrary rate interpolator can be used to align the sampling clock with either timing or carrier sub systems.
When describing the processing technique presented herein we will use sample rates selected to satisfy the timing consideration. The sample rate indicated in FIG. 1 has been selected to demonstrate the high computational efficiency available for timing recovery from the process described herein.
The tasks and the associated modules that implement the functions in a traditional receiver are: (1) Spectral Translation, (2) Bandwidth Reduction, and (3) Output Sample Rate Selection. The Spectral Translation is performed by a complex heterodyne that translates the center of the desired spectral band to base-band. The complex heterodyne multiplies the input data sequence by samples of a cosine wave and a sine wave with frequency selected to match the center of the desired band. The Bandwidth Reduction is performed by a digital filter that processes the complex input data stream of the down-converted signal. The digital filter performs the required weighted sums to form the reduced bandwidth output data stream. The digital filter performs a low-pass filtering process that restricts the signal bandwidth to that of the translated band, and consequently rejects the remaining spectral components of the translated signal. The Output Sample Rate Selection is performed by a complex digital filter known as an interpolator that accept input data from the previously described low-pass filter at a fixed input rate that satisfies the Nyquist Criterion, and computes from these samples a set of output samples at an output rate different from the input rate and selected to satisfy some signal conditioning constraint in subsequent processing following this processing block.